1. Field of the Invention
The present invention relates to a cleaning blade and an image forming apparatus, and particularly relates to a cleaning blade for removing residual toner from a surface of a toner image carrier and an image forming apparatus such as an electrophotographic copier or printer.
2. Description of Related Art
In recent years, in an electrophotographic image forming apparatus, improvements in image quality such as a higher resolution and higher photograph reproducibility have been demanded, and as an effective measure to meet the demand, size reduction or conglobation of toner particles has been proposed.
However, toner particles of a reduced size or conglobated toner particles are likely to pass through a space between a cleaning blade and a toner image carrier such as a photoreceptor or an intermediate transfer member. The reasons are as follows. A size reduction of toner particles strengthens the van der Waals force and accordingly strengthens the adhesive force between the toner and the image carrier, and toner particles of a reduced size are likely to get into a nip portion between the image carrier and the cleaning blade. Further, conglobated toner particles are likely to roll between the image carrier and the cleaning blade, and the rolling toner particles get into the nip portion easily.
When the toner passes through the cleaning blade (when a cleaning failure occurs), the toner that passed through the cleaning blade may be transferred to a subsequently formed image to cause black streaky noise, or the toner that passed through the cleaning blade may block light in a subsequent exposure process to cause a partial defect of an electrostatic latent image. Therefore, it is important to prevent the cleaning failure.
In order to ensure toner cleaning be successfully conducted, there has been taken a measure of not conglobating toner particles much but holding toner particles in some degree of amorphous state. Moreover, toner particles are coated with inorganic minute particles, called a cleaning aid, with a relatively large size of the order of several hundreds of nanometers to one micron meter. When a developer including toner particles coated with such a cleaning aid is stirred in a development unit, part of the cleaning aid is separated from the toner particles and charged. The separated part of the cleaning aid is then used for development, separately from the toner, and fed to a cleaning section. Also, there is some other part of the cleaning aid fed to the cleaning section along with the toner particles. The cleaning aid fed along with the toner particles is separated from the toner particles in a toner gathering portion formed by the blade.
The cleaning aid fed to the toner gathering portion of the blade functions in the following way. As shown in FIG. 6, the toner gathering portion formed by a cleaning blade 114 is in the shape of a wedge formed of the surface of an image carrier (photoreceptor 110) and the cleaning surface of the blade 114, and is closed at the downstream edge in a traveling direction of the photoreceptor 110 (see arrow a). Particles of the toner and the cleaning aid gathered in this wedge are arranged according to the particle size, in consequence of the travel (rotation) of the photoreceptor 110. Specifically, the smaller size the particle has, the closer to the tip of the wedge the particle is located. Accordingly, particles of the cleaning aid, which have smaller sizes than particles of the toner, are gathered in a deeper portion in the wedge than the toner. Then, some of the cleaning aid particles slip into the nip section between the blade 114 and the photoreceptor 110, due to the small particle size.
However, the cleaning aid particles that slipped into the nip section between the photoreceptor 110 and the blade 114 reduce the frictional force of the nip section, thereby preventing a rebound of the blade 114 and suppressing wear of the edge of the blade 114. Thus, the cleaning aid serves as a lubricant. Since the cleaning aid is continuously fed from the development unit to the surface of the photoreceptor 110 or is separated from the toner in the toner gathering portion, the cleaning aid that slipped into the nip section is compensated with the fed cleaning aid, and there is always a certain amount of cleaning aid in the toner gathering section. Then, the cleaning aid gathered in the gathering section blocks the toner from coming into the nip section. In other words, the toner is certainly cleaned.
Further, in order to facilitate cleaning of the toner, there has been proposed a measure of supplying a material onto the image carrier to lower the friction coefficient of the image carrier. For example, a solid lubricant, specifically, solidified zinc stearate is pressed against a rotary-driven brush and is scraped off by the brush, and the scraped lubricant is applied to the image carrier. The lubricant applied to the image carrier reduces the adhesive force of toner to the image carrier and reduces the frictional force between the toner and the image carrier, thereby allowing a cleaning blade or the like to remove the toner satisfactorily.
However, a large number of executions of image formation in the state where zinc stearate is sufficiently applied to the image carrier to reduce the friction coefficient causes a problem in that the cleaning blade is worn to a more serious degree, compared with a case where zinc stearate is not applied, even though the application of zinc stearate to the image carrier has an effect on suppression of the wear of the image carrier. This is caused by the following actions. Since the image carrier and the cleaning blade are rubbed against each other, there is a relation between the wear of the image carrier and the wear of the cleaning blade. A typical image carrier in use in recent years is an organic photoreceptor, and the portion of the organic photoreceptor to rub against the cleaning blade is mainly made of polycarbonate resin. For the cleaning blade, conventionally, polyurethane rubber has been used. Polycarbonate and urethane rubber are different in hardness, and specifically, polycarbonate is harder than urethane rubber. Therefore, when polycarbonate and urethane rubber are rubbed against each other, urethane rubber is worn.
However, as described above, the cleaning aid is present between the image carrier and the cleaning blade, and the cleaning aid constantly slips into the cleaning nip section. For this reason, the slip-in of the cleaning aid has great bearing on the wear of the image carrier and the wear of the cleaning blade. The cleaning aid particles that come into the nip section prevent the cleaning blade and the image carrier from coming into direct contact with each other, and further, the cleaning aid particles rolling into the nip section reduces the frictional force. Therefore, the wear of the edge of the cleaning blade in this case is significantly smaller than the wear of the edge of the cleaning blade that is caused by direct contact with the image carrier.
Meanwhile, the cleaning aid is likely to serve as an abrasive on the image carrier. Therefore, when no cleaning aid slips into the nip section, the wear of the image carrier is at the minimum degree. As the amount of cleaning aid slipping into the nip section becomes greater, the wear of the image carrier becomes to a greater degree. When a lubricant is applied to the image carrier to decrease the friction coefficient, a less amount of the cleaning aid slips into the nip section, and it becomes more likely that the image carrier and the cleaning blade come into direct contact with each other. This precipitates the wear of the cleaning blade. In this case, also, a decrease in the amount of cleaning aid slipping into the nip section leads to an increase in the frictional force, and the edge of the blade is pulled farther by the rotating image carrier and curves more. This accelerates the wear of the cleaning blade.
Moreover, when the edge of the blade curves more due to an increase in the frictional force, the blade is likely to rebound drastically and suddenly. The rebound of the blade causes not only a cleaning failure but also a damage on the image carrier. In this case, also, since the rotating image carrier pulls the edge of the blade strongly, the drive torque for rotating the image carrier increases extremely. Then, the increase in the drive torque may result in a breakdown of the driving device. Therefore, it is more important to prevent the rebound of the blade than to prevent the cleaning failure due to the wear of the edge of the blade.
In order to deal with the above problem, there has been considered a measure of heightening the hardness of the cleaning blade. When the hardness of the cleaning blade is heightened, the width of the edge of the cleaning blade to form the nip section (the width being hereinafter referred to as a nip width) is decreased. Since the frictional force is proportional to the contact area, the decrease in the nip width of the edge leads to a decrease in the frictional force. Further, the cleaning blade with high hardness is improved in wear resistance. The decrease in the frictional force and the improvement in the wear resistance have synergistic effect to suppress the wear of the cleaning blade. Moreover, the cleaning blade with high hardness is less likely to rebound, and the rebound of the blade can be prevented.
However, the cleaning blade with high hardness is unfavorable in view of permanent distortion, and is difficult to hold the initial contact state. That is, although heightening the hardness of the blade leads to prevention of the rebound of the blade and suppression of the wear of the edge of the blade, the blade with high hardness is difficult to hold an appropriate state to contact with the image carrier, which causes a cleaning failure. Thus, a problem in terms of the length of life of the cleaning blade is remained to be solved.
In order to achieve a cleaning blade with suppressed permanent distortion, improved wear resistance and less incidence of rebound, two-layer blade structures have been proposed. According to the structures, a blade is formed by laminating a contact layer including an edge made of urethane rubber and a supporting layer that supports the contact layer. A material with high hardness has been adopted for the contact layer to improve the wear resistance and to prevent the rebound, and a material with low hardness has been adopted for the supporting layer to suppress the permanent distortion (Japanese Patent Application Laid-Open Nos. 2008-268302, 2009-031773 and 2010-181718).
However, according to a study by the present inventors, it has been confirmed that physical properties of the material for the supporting layer are related to the wear and the rebound of the contact layer. It has been revealed that a two-layer blade with a supporting layer simply formed of a material with low hardness does not necessarily have improved wear resistance and less incidence of rebound and rather possibly has deteriorated wear resistance and more incidence of rebound than a single-layer blade with only a contact later.